Project Summary Depression is an important problem after traumatic brain injury (TBI), and evidence-based treatments are lacking. To develop new treatments and to target existing therapies to appropriate individuals, it is important to understand the longitudinal trajectories of depression following TBI as well as its relationship to person characteristics, injury characteristics, and situational factors. This project will use a large (n = 2,330) cohort from the Traumatic Brain Injury Model System National Database: participants who have completed the PHQ-9 measure of depressive symptoms at 1 and 2 years after injury. Growth Mixture Modeling (GMM) will be used to characterize subgroups of patients that differ in the association (joint trajectory) between depression and societal participation over time. In secondary analyses we will explore the specific contributions of three sub-types of participation: social relations, community activity, and productivity. The ultimate aim of this research is to elucidate for whom depression follows limitations in valued activity, for whom the reverse relationship might occur, and what other factors bear on the relationship between participation and depression following TBI. New knowledge about contributors to depression after TBI will lead directly to the development of treatments, or adaptation of treatments with a strong evidence base for depression in the general population, targeted to the persons most likely to benefit.